You forgot the option of a fire... BURN, GREAT VALLEY, BURN!!!
I could see a really sinister villain set it on fire on purpose... now that'd make for a dark movie, and an exciting one at that!
Funny you should mention that. I was going to have Sierra and his evil flyers do that in my fanfic movie XX: The Heart Whispers. It was a plan to starve them out so that they would have to either surrender to the Bothteeth or run out of the Valley right into Red Claw's clutches.
Luckily, Chomper has an idea and removes lots of boulders and causes water to come in and put it out, and it has the added benefit of also washing away bad Sharpteeth that got into the Great Valley.